A stopper for stopping rotation of a planar element with respect to a frame when the planar element is hinged to the frame, the stopper has a longitudinal axis and comprises: a locking bar having a toothed surface and received within an indentation, an operation handle for moving the locking bar out of the indentation, a locking spring having a locking tooth that is urged into the toothed surface, a releasing lever for releasing the locking tooth from the toothed surface, a retention spring for urging the locking bar into the indentation. A linear movement of the operation handle moves a portion of the locking bar out of the indentation, the locking bar abuts against the frame, and, the locking bar is fixed in its position by means of the locking tooth that is urged against the toothed surface.